This invention relates generally to thermally responsive circuit interrupting devices and more particularly to thermally actuated switches having contractable solid elements biased for circuit-opening movement by a spring force and held in a conductive condition by masses of fusible material and method of manufacture of such thermally actuated switches.
Examples of devices which are intended to open electric circuits whenever the temperature surrounding the devices reaches a predetermined level are shown in U.S. Pat. Nos. 2,805,304; 2,913,555; 3,301,981; and 4,032,877. These devices include a housing designed to allow travel of a nonfusible circuit-completing conductor from its normal closed-circuit position to an open-circuit position. Such conductors are characteristically biased for circuit-opening movement by compressed helical springs which, when released, expand into cavities within the housings. Devices of the aforementioned type characteristically employ a separate circuit-completing conductor and spring biasing means.
Other devices which are intended to open electric circuits when the temperature in the immediate vicinity of the device reaches a predetermined level are disclosed in U.S. Pat. Nos. 3,924,218; 4,259,656; 4,276,531; and 4,276,532. These devices are characterized by the use of thermally sensitive pellets. The thermally sensitive pellets generally are of a nonconductive material, to avoid short circuiting the device upon melting. The pellets occupy a relatively large volume within the housing of the device to allow sufficient travel of the opening contacts, thereby to insure complete clearing.
While the prior art spring operated, thermally actuated switches operate satisfactorily for the most part, many are costly to manufacture and generate unnecessarily great amounts of internal heat, making their temperature ratings less precise.